Conveyor systems for transporting workpieces by means of carriers along a transport path are known. The carriers are designed for example to carry workpieces and are moved in guides along a typically closed or annular transport path. As the carriers travel around the path they move the respective workpieces they carry through a plurality of treatment stations integrated with the transport path.
These work stations can for example be treatment stations of a printing machine, such as loading and removing stations, printing mechanisms, lacquering mechanisms, drying units, identification units and the like. The workpieces can for example be any type of objects which are to be printed, such as cards, data media—CD's, DVD's—or the like. A conveyor system can thus form a part of a printing machine but also be used for any other application in which workpieces are to be transported.
Transporting the carriers in the known conveyor systems is done for example in that the carriers are attached to circumferential chains or conveyor belts or are at least in sections are moved along by them in the carriers by respective grippers or pushers, the conveyor chains or the conveyor belts being driven at least in sections by a common drive.
This is disadvantageous in that the carriers at least in sections invariably have the same displacement mode, such as all moving continuously or in steps. Processing stations that require different work movements or displacement modes of the carriers can thus not be provided along the path, or at least on the path near another station requiring a different displacement mode. If this is required for one production sequence though, then it is necessary to correspondingly combine a plurality of conveyors working with different work movements into one joint conveyor system.
Hence, the carriers have to be moved smoothly between conveyor transport-path sections that work with a different displacement modes. However, integrating such systems presents a number of problems, since the respective adjacent conveyor units have to work precisely synchronous with each other during the transfer of a carrier to a downstream conveyor, in order to avoid vibration, impacts and the like to the carriers and thus to the workpieces, since they can severely disrupt the production process.
Furthermore, it is possible only with difficulty, particularly with mechanical delivery of the carriers between succeeding conveyors that are working differently, to make later adjustments or process changes in the work movement, such that a conveyor system of this kind has limited flexibility.
It is furthermore disadvantageous that the necessary positioning accuracy of a carrier within a treatment station is guaranteed exclusively by the conveyor system only within limits, since only average accuracy can be achieved due to workpiece tolerances, conveyor tolerances and wear. Accuracy will deteriorate in the course of operations.
In order to achieve the required accuracy within a treatment station, it is therefore usually necessary to uncouple the carrier with the workpiece attached to it from the conveyor at least for a short time during treatment and to align it accordingly with external devices. After completed treatment, the carrier must then be reconnected to the conveyor. Obviously such a procedure can only function within a substantially inflexible functional sequence in order to suppress the above-mentioned disturbing influences.
For this reason it is virtually impossible once the system is set up and operating to make changes in the functional sequence for the optimizing or for implementing other or new treatments into an existing functional sequence and thus into an existing conveyor system.
In another embodiment of known conveyor systems, the carriers are moved for example by means of a screw conveyor subdivided into sections along the transport path, in that one or a plurality of pusher elements attached to the carriers engage in one or a plurality of screwthreads of the screw conveyor and are moved along during the rotation of the screw conveyor via the pitch of the screwthreads. Different transportation speeds of the carriers can be achieved depending on the embodiment of the pitch profiles along the individual screws.
This type of conveyor system has the disadvantage that this type of movement invariably only allows an unalterable speed profile, depending substantially on the design of the screw. It is furthermore unfavorable that this type of conveyance is possible merely within a straight section of the entire transport path, so that delivery to a conveyor system that is working differently has to take place at each end of a conveyor screw, which is usually complex and problematic, as already explained.